Sound field spatial stabilizer with structured noise compensation

ABSTRACT

In a system and method for maintaining the spatial stability of a sound field a balance gain may be calculated for two or more microphone signals. The balance gain may be associated with a spatial image in the sound field. Signal values may be calculated for each of the microphone. The signal values may be signal estimates or signal gains calculated to improve a characteristic of the microphone signals. The differences between the signal values associated with each microphone signal may be limited although some difference between signal values may be allowable. One or more microphone signals are adjusted responsive to the two or more balance gains and the signal gains to maintain the spatial stability of the sound field. The adjustments of one or more microphone signals may include mixing of two or more microphone. The signal gains are applied to the two or more microphone signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure refers to:

U.S. patent application Ser. No. 13/753,198, titled “Sound Field SpatialStabilizer”, filed Jan. 29, 2013; andU.S. patent application Ser. No. 13/753,162, titled “Noise EstimationControl System”, filed Jan. 29, 2013.

Each of the above identified patent applications is hereby incorporatedherein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to the field of processing sound fields.In particular, to a system and method for maintaining the spatialstability of a sound field.

2. Related Art

Stereo and multichannel microphone configurations may be used forprocessing a sound field that is a spatial representation of an audibleenvironment associated with the microphones. The audio received from themicrophones may be used to reproduce the sound field using audiotransducers.

Many computing devices may have multiple integrated microphones used forrecording an audible environment associated with the computing deviceand communicating with other users. Some computing devices use multiplemicrophones to improve noise performance with noise suppressionprocesses. The noise suppression processes may result in the reductionor loss of spatial information. In many cases the noise suppressionprocessing may result in a single, or mono, output signal that has nospatial information.

BRIEF DESCRIPTION OF DRAWINGS

The system may be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the disclosure. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

Other systems, methods, features and advantages will be, or will become,apparent to one with skill in the art upon examination of the followingfigures and detailed description. It is intended that all suchadditional systems, methods, features and advantages be included withthis description, be within the scope of the invention, and be protectedby the following claims.

FIG. 1 is a schematic representation of a system for maintaining thespatial stability of a sound field.

FIG. 2 is a further schematic representation of a system for maintainingthe spatial stability of the sound field.

FIG. 3 is a schematic representation of another system for maintainingthe spatial stability of a sound field when reproduced in an outputsound field.

FIG. 4 is a further schematic representation of a system for maintainingthe spatial stability of the sound field.

FIG. 5 is a further schematic representation of a system for maintainingthe spatial stability of the sound field.

FIG. 6 is a representation of a method for maintaining the spatialstability of the sound field.

FIG. 7 is a further schematic representation of a system for maintainingthe spatial stability of the sound field.

FIG. 8 is a representation of a method for maintaining the spatialstability of the sound field.

FIG. 9 is a further schematic representation of a system for maintainingthe spatial stability of the sound field.

FIG. 10 is a representation of a method for maintaining the spatialstability of the sound field.

FIG. 11 is a further schematic representation of a system formaintaining the spatial stability of the sound field.

DETAILED DESCRIPTION

In a system and method for maintaining the spatial stability of a soundfield balance gains may be calculated for each of two or more microphonesignals. The balance gain may be associated with a spatial image in thesound field. One or more signal values may be calculated for each of thetwo or more microphone signals. The signal values may be the backgroundnoise estimate or signal gains associated with echo cancellation andnoise reduction processes. Structured noise content may be detected foreach of the two or more microphone signals. The structured noise contentmay be for example, impulse noise or tonal noise. A first microphonesignal of the two or more microphone signals may be mixed with a secondmicrophone signal of the two or more microphone signals responsive tothe detected structured noise. Increasing amounts of detected structurednoise may increase the amount of mixing, or blending, of the firstmicrophone signal with the second microphone signal. The gain may beadjusted for the two or more microphone signals, including the mixedfirst microphone signal and second microphone signal, responsive to thecalculated balance gains and the one or more signal values for each ofthe two or more microphone signals.

In a system and method for maintaining the spatial stability of a soundfield balance gains may be calculated for each of two or more microphonesignals. The balance gain may be associated with a spatial image in thesound field. One or more signal values may be calculated for each of thetwo or more microphone signals. The signal values may be the backgroundnoise estimate or signal gains associated with echo cancellation andnoise reduction processes. A pair-wise spectral coherence may becalculated between each of the two or more microphone signals. Thepair-wise spectral coherence may indicate that two or more microphonesignals are correlated and may have captured a signal of interest. Thetwo or more microphone signals may be gain adjusted responsive to thecalculated balance gains, the one or more signal values, and thepair-wise spectral coherence for each of the two or more microphonesignals. The spectral coherence value may be used to prevent highamplitude high frequencies signals from being unnecessarily attenuatedand may also be used to increase the gain of low amplitude highfrequency signals.

In a system and method for maintaining the spatial stability of a soundfield balance gains may be calculated for each of two or more microphonesignals. The balance gain may be associated with a spatial image in thesound field. One or more signal values may be calculated for each of thetwo or more microphone signals. The signal values may be the backgroundnoise estimate or signal gains associated with echo cancellation andnoise reduction processes. A predicted echo may be calculated for areceived audio signal. The predicted echo may be used to reduce an echosignal. A pair-wise echo spectral coherence may be calculated betweenthe predicted echo and the two or more microphone signals. The pair-wiseecho spectral coherence may indicate that the predicted echo iscorrelated to one or more of the captured two or more microphonesignals. A pair-wise spectral coherence between each of the two or moremicrophone signals. The pair-wise spectral coherence may indicate thattwo or more microphone signals are correlated and may have captured asignal of interest. The two or more microphone signals may be gainadjusted responsive to the calculated balance gains, the one or moresignal values, the echo spectral coherence and the pair-wise spectralcoherence for each of the two or more microphone signals. Using both ofthe echo spectral coherence and the spectral coherence values in orderto adjust the signal gains may reduce the noise artifacts, preserve andenhance the signal of interest, and reduce the echo.

FIG. 1 is a schematic representation of a system for maintaining thespatial stability of a sound field 100. Two or more microphones 102receive the sound field. Stereo and multichannel microphoneconfigurations may be utilized for processing the sound field that is aspatial representation of an audible environment associated with themicrophones 102. Many audible environments associated with themicrophones 102 may include undesirable content that may be mitigated byprocessing the received sound field. Microphones 102 that are arrangedin a far field configuration may receive more undesirable content,noise, than microphones 102 in a near field configuration. Far fieldconfigurations may include, for example, a hands free phone, aconference phone and microphones embedded into an automobile. Far fieldconfigurations are capable of receiving a sound field that representsthe spatial environment associated with the microphones 102. Near fieldconfigurations may place the microphone 102 in close proximity to auser. Undesirable content may be mitigated in both near and far fieldconfigurations by processing the received sound field.

Processing that may mitigate undesirable content received in the soundfield may include echo cancellation and noise reduction processes. Echocancellation, noise reduction and other audio processing processes maycalculate one or more suppression, or signal, gains utilizing asuppression gain calculator 106. An echo cancellation process and anoise reduction process may each calculate one or more signal gains.Each respective signal gains may be applied individually or a compositesignal gain may be applied to process the sound field using a gainfilter 114. Echo cancellation processing mitigates echoes caused bysignal feedback between two or more communication devices. Signalfeedback occurs when an audio transducer on a first communication devicereproduces the signal received from a second communication device andsubsequently the microphones on the first communication device recapturethe reproduced signal. The recaptured signal may be transmitted to thesecond communication device where the recaptured signal may be perceivedas an echo of the previously transmitted signal. Echo cancellationprocesses may detect when the signal has been recaptured and attempt tosuppress the recaptured signal. Many different echo cancellationprocesses may mitigate echoes by calculating one or more signal gainsthat, when applied to the signals received by the microphones 102,suppress the echoes. In one example implementation, the echo suppressiongain may be calculated using coherence calculation between the predictedecho and the microphone disclosed in U.S. Pat. No. 8,036,879, which isincorporated herein by reference, except that in the event of anyinconsistent disclosure or definition from the present specification,the disclosure or definition herein shall be deemed to prevail.

When the microphone 102 and an audio transducer are close in proximity,the echo cancellation process may determine that a large amount ofsuppression, or calculate large signal gains, as a result of the signalproduced by the audio transducer dominating, or coupling with, themicrophone 102.

When one of the microphones 102 and an audio transducer are in closeproximity, the echo cancellation process may determine that a largeamount of suppression may mitigate the signal produced by the audiotransducer from dominating or coupling with, the microphone 102. Theecho cancellation process may calculate large signal gains to mitigatethe coupling. The large signal gains may result in a gating effect wherethe communication device effectively supports only half duplexcommunication. Half duplex communication may occur when thecommunication channel allows for reliable communication fromalternatively either the far side or near side but not bothsimultaneously. The large signal gains may suppress the coupling but mayalso suppress all content, including desired voice content resulting inhalf duplex communication.

Background noise is another type of undesirable signal content that maybe mitigated by processing the received sound field. Many differenttypes of noise reduction processing techniques may mitigate backgroundnoise. An exemplary noise reduction method is a recursive Wiener filter.The Wiener suppression gain G_(i,k), or signal gain, is defined as

$\begin{matrix}{G_{i,k} = {\frac{S\; \hat{N}\; R_{{priory}_{i,k}}}{{S\; \hat{N}\; R_{{priori}_{i,k}}} + 1}.}} & (1)\end{matrix}$

Where S{circumflex over (N)}R_(priori) _(i,k) is the a priori SNRestimate and is calculated recursively by

S{circumflex over (N)}R _(priori) _(i,k) =G _(i-1,k) S{circumflex over(N)}R _(post) _(i,k) −1.  (2)

S{circumflex over (N)}R_(post) _(i,k) is the a posteriori SNR estimategiven by

$\begin{matrix}{{S\; \hat{N}\; R_{{post}_{i,k}}} = {\frac{{Y_{i,k}}^{2}}{{{\hat{N}}_{i,k}}^{2}}.}} & (3)\end{matrix}$

Here |{circumflex over (N)}_(i,k)| is a background noise estimate. Inone example implementation, the background noise estimate, or signalvalues, may be calculated using the background noise estimationtechniques disclosed in U.S. Pat. No. 7,844,453, which is incorporatedherein by reference, except that in the event of any inconsistentdisclosure or definition from the present specification, the disclosureor definition herein shall be deemed to prevail. In otherimplementations, alternative background noise estimation techniques maybe used, such as, for example, a noise power estimation technique basedon minimum statistics.

Additional noise reduction processing may mitigate specific types ofundesirable noise characteristics including, for example, wind noise,transient noise, rain noise and engine noise. Mitigation of somespecific types of undesirable noise may be referred to as signaturenoise reduction processes. Signature noise reduction processes detectsignature noise and generate signal gains that may be used to suppress adetected signature noise. In one implementation, wind noise suppressiongains (a.k.a. signal gains) may be calculated using the system forsuppressing wind noise disclosed in U.S. Pat. No. 7,885,420, which isincorporated herein by reference, except that in the event of anyinconsistent disclosure or definition from the present specification,the disclosure or definition herein shall be deemed to prevail.

The sound field received by the two or more microphones 102 may containa spatial representation, or a spatial image, of an audible environment.Balance gains may be calculated responsive to the spatial image in thesound field. The balance gains may be calculated with a balancecalculator 108. The balance calculator 108 may calculate the balancegains by measuring an energy level in a signal from each microphone 102.The energy level differences may represent the approximate balance ofthe spatial image. One or more energy levels may be calculated for eachmicrophone 102 generating one or more balance gains. A single balancegain may be utilized in a two microphone configuration where the singlebalance gain may be the ratio of energy levels between the twomicrophone signals 118.

A subband filter may process the received microphone signal 118 toextract frequency information. The subband filter may be accomplished byvarious methods, such as a Fast Fourier Transform (FFT), critical filterbank, octave filter band, or one-third octave filter bank.Alternatively, the subband analysis may include a time-based filterbank. The time-based filter bank may be composed of a bank ofoverlapping bandpass filters, where the center frequencies havenon-linear spacing such as octave, 3^(rd) octave, bark, mel, or otherspacing techniques. The one or more energy levels may be calculated foreach frequency bin or band of the subband filter. The resulting balancegains may be filtered, or smoothed, over time and/or frequency. Thebalance calculator 108 may update the balance gains responsive todesired signal content. For example, the balance gains may be updatedwhen, for example, the energy level exceeds a threshold, the signal tonoise ratio (SNR) exceeds a threshold, a voice activity detector detectsvoice content or any combination thereof.

The background noise estimator 104 may calculate a background noiseestimate, or signal value, for each microphone signal 118. When themicrophones 102 are spaced apart, the background noise estimator 104 maycalculate different signal values responsive to the received soundvalue. Some difference in the calculated background noise estimate maybe acceptable but relatively large differences may indicate a potentialcorruption or misrepresentation of one or more of the signals. Forexample, a user may be blocking one microphone 102 with a fingerresulting in a relatively large difference in the background noiseestimate. The background noise estimate may be utilized for manysubsequent calculations including signal-to-noise ratios, echocancellers and noise reduction calculators. When the subsequentcalculations utilize background noise estimates that contain relativelylarge differences the subsequent calculations may yield corrupted ormisrepresentative results. For example, large differences in suppressiongains between microphones 102 may result in audible distortions in thespatial image of the sound field.

A difference limiter 110 may limit the difference in the backgroundnoise estimates, or signal values, and/or the adaption rates utilized inthe background noise estimator 104. The different limiter 110 maymitigate audio distortions in the spatial image when reproduced in theoutput sound field. For example, a difference between correspondingsignal values in the calculated background noise estimates may beacceptable when the difference is about 2 dB (decibels) to about 4 dBbut noticeable when the difference exceeds about 6 dB. The differencelimiter 110 may, for example, limit the difference between signal valuesto about 6 dB or may allow a difference proportional to the signal valuewhen the difference is greater than about 6 dB. The difference limiter110 may utilize a coherence and/or correlation calculation betweenmicrophones to limit a difference between the signal values. Two signalsthat are correlated may indicate that the difference between signalvalues should be limited. The difference limiter 110 may smooth, orfilter, the amount of limiting over time and frequency.

The difference limiter 110 may be applied to other signal valuesincluding suppression gains, or signal gains, calculated using thesuppression gain calculator 106. The suppression gain calculator 106 maycalculate signal gains for the echo cancellation and noise reductionprocesses described above. Signature noise reduction processes maycalculate signal gains that have large differences between microphonesignals 118. For example, in the case of wind noise reduction, a firstmicrophone 102 may receive significant wind noise and the secondmicrophone 102 may receive negligible wind noise. An example portablecomputing device may have two microphones 102 placed several inchesapart where the first microphone 102 may be located on the bottomsurface and the second microphone 102 may be located on the top surface.The first microphone 102 and the second microphone 102 may be relativelyclose in position although they may not be close enough to process phasedifferences to utilize, for example, a beam forming combining process.Even though the microphones 102 are relatively close in position on theexample portable computing device, one microphone 102 may receivesignificant wind noise. The suppression gain calculator 106 maycalculate signal gains that may contain relatively large differences.The difference limiter 110 may allow some of the wind noise to besuppressed while mitigating audio distortions in the spatial image ofthe sound field. For example, a difference between corresponding signalgains generated by the suppression gain calculators 106 may beacceptable when the difference is about 2 dB to 4 about dB butnoticeable when the difference exceeds about 6 dB. The differencelimiter 110 may limit the difference between signal values to 6 about dBor may allow a difference proportional to the signal value when thedifference is greater than 6 dB. The difference limiter 110 may smooth,or filter, the amount of limiting over time and frequency.

The difference limiter 110 may mitigate some distortion in the spatialimage when reproduced in the output sound field although it may bepossible that the combination of one or more of the signal valuescalculated utilizing the background noise estimator 104 and suppressiongain calculator 106 may still distort the spatial image. Additionally,in some cases the suppression gain calculator 106 may not utilize thedifference limiter 110. For example, when the microphone 102 and audiotransducer are coupled as described above resulting in a gating effect,the difference limiter 110 may not be utilized because the audibleartifacts associated with the coupling are perceptibly more distractingthan distorting the spatial image. In this case, the echo cancellationprocess may be allowed to gate the microphone signal 118 withoutapplying the difference limiter 110.

A balance adjuster 112 may maintain the spatial stability whenreproduced in the output sound field. The balance adjuster 112 maymitigate distortions in the spatial image that may not be mitigated withthe difference limiter 110. Additionally, the balance adjuster 112 maymitigate audio distortions in the spatial image where the differencelimiter 110 may not be applied. The balance adjuster 112 may adjust thesignal gains using the balance gains calculated with the balancecalculator 108 and the signal gains. The balance gains may represent theapproximate balance of the spatial image. The balance adjuster 112 mayadjust the signal gains responsive to the balance gains. Additionally,the balance adjuster 112 may mix, or borrow, between two or moremicrophone signals 118 to maintain the spatial stability and to moreclosely track the balance gains. In one example, the echo-gatingtriggered half-duplex use case described above may have a firstmicrophone signal 118 that may be gated. The balance adjuster 112 maymitigate audio distortions in the spatial image by borrowing audio froma second microphone signal 118 responsive to the balance gain. Thesecond microphone signal 118 may have associated signal gains that maybe adjusted responsive to the balance gain. The second microphone signal118 that is borrowed may be mixed into the first microphone signal 118.The balance adjuster 112 may adjust the signal gains and the borrowingof microphone signals 118 may be filtered, or smoothed, over time andfrequency. The adjustments may be performed on a frequency bin and/orband using the subband filter described above.

A gain filter 114 applies the signal gains to the two or more microphonesignals 118. The signal gains may be a combination of signal gainsassociated with one or more suppression gain calculators 106. The gainfilter 114 may utilize the subband filter described above.

FIG. 2 is a schematic representation of a further system for maintainingthe spatial stability of a sound field when reproduced in an outputsound field. The system of FIG. 2 may provide the same or similarfunctionality as the system described with reference to FIG. 1. FIG. 2does not show the microphones 102 and the background noise estimator 104but they may be included in the system 200. The system 100 in FIG. 1 maybe able to reduce common audio noise artifacts such as wind noise whentwo or more microphones 102 capture a similar voice of interest. One ofthe microphones 102 may capture more of the example wind noise thanother microphones 102. The gain of a higher amplitude microphone signal118 may be brought down, or reduced, to a lower amplitude microphonesignal 118, on a frequency bin-by-frequency bin basis, and to the extentto which the microphone signals 118 are “unbalanced”. Small differencesbetween microphone signals 118 may be normal so no adjustment is made. Alarge difference may not be normal and may result in a maximum amount ofgain reduction on the higher amplitude microphone signal 118.

The system 200 adds processing components relative to the system 100where gain reduction alone may not be able to remove the noiseartifacts. Some noise artifacts, including impulses and tonal noises,may still be audible even after the gain has been reduced on the higheramplitude microphone signal 118. These types of noise artifacts, orstructured noise, may have all the information stored in their phase.For example, an impulse has energy at all frequencies, and the phase atall frequencies is aligned so that the energy is delivered at one pointin a time-series train. Reducing the gain of a microphone signal 118containing an impulse may only result in making the impulse quieter. Thesystem 200 includes a channel mixer 204 to blend the higher amplitudemicrophone signal 118 with the lower amplitude microphone signal 118,responsive to the amount of structured noise in the higher amplitudemicrophone signal 118. A maximum reduction of the high amplitudemicrophone signal 118 may take the form of a full copy of the lowamplitude microphone signal 118. The blending, or mixing, may beperformed on a frequency bin-by-frequency bin basis so that when thehigher amplitude microphone signal 118 contains tonal noise, andtherefore may be confined to one or two frequency bins, only thosefrequency bins are affected. Blending the higher amplitude microphonesignal 118 with the lower amplitude microphone signal 118 may reducestructured noises that occur during voice content with minimal impact tothe voice content.

A structured noise detector 202 detects structured noise artifacts,including impulse noise and tonal noise, in two or more microphonesignals 118. In one implementation, transient noise may be detectedusing the system for repetitive transient noise removal disclosed inU.S. Pat. No. 8,073,689, which is incorporated herein by reference,except that in the event of any inconsistent disclosure or definitionfrom the present specification, the disclosure or definition hereinshall be deemed to prevail. In one implementation, tonal noise may bedetected using the system for noise reduction with integrated tonalnoise reduction disclosed in U.S. Publication No. 2008/0167870, which isincorporated herein by reference, except that in the event of anyinconsistent disclosure or definition from the present specification,the disclosure or definition herein shall be deemed to prevail.Alternatively, the structured noise detector 202 may indicate noisecontent when the amplitude of a first microphone signal 118 exceeds athreshold when compared to the amplitude of a second microphone signal118. The channel mixer 204 may be responsive to the outputs of thestructured noise detectors 202 to blend the higher amplitude microphonesignal 118 with the lower amplitude microphone signal 118, responsive tothe amount of structured noise in the higher amplitude microphone signal118. An increasing amount of structured noise detected in the structurednoise detector 202 may blend more of the lower amplitude microphonesignal 118 with the higher amplitude microphone signal 118. A thirdmicrophone signal 118 with higher amplitude may blend more of the loweramplitude microphone signal 118 or a combination of lower amplitudemicrophone signals 118. A maximum reduction of the high amplitudemicrophone signal 118 may take the form of a full copy of the lowamplitude microphone signal 118. For example, when the high amplitudemicrophone signal 118 contains a strong impulse detected by thestructured noise detector 202, the channel mixer may copy the contentsof the lower amplitude microphone signal 118 to the high amplitudemicrophone signal 118. The channel mixer 204 may adjust the gain of theblended microphone signal 118 responsive to, for example, matching afiltered, or smoothed, energy level over time.

A gain adjuster 206 may adjust the signal gains 208 using the balancegains 210 calculated with the balance calculator 108 and the signalgains 208. The gain adjuster 206 may perform similarly to the balanceadjuster 112 described above in FIG. 1. The adjusted signal gains 208are applied to each of the blended two or more microphone signal 118using the gain filter 114. The signal gains 208 may be a combination ofsignal gains 208 associated with one or more suppression gaincalculators 106. The gain filter 114 may utilize the subband filterdescribed above.

FIG. 3 is a schematic representation of another system for maintainingthe spatial stability of a sound field when reproduced in an outputsound field. The system of FIG. 3 may provide the same or similarfunctionality as the systems described with reference to FIG. 1 and FIG.2. FIG. 3 does not show the microphones 102, the background noiseestimator 104, the structured noise detector 202, the channel mixer 204and the gain adjuster 206 but they may be included in the system 300.The system 300 may include a coherence calculator 302 that calculates apair-wise spectral coherence between two or more microphone signals 118.In the case of two microphone signals 118 including a left and a rightmicrophone signal 118 the spectral coherence may be referred to asCohLR. In one implementation, the spectral coherence CohLR may becalculated in a similar fashion to that of CohDY using the system fornoise estimation control disclosed in U.S. patent application Ser. No.13/753,162, which is incorporated herein by reference, except that inthe event of any inconsistent disclosure or definition from the presentspecification, the disclosure or definition herein shall be deemed toprevail. The result of the spectral coherence calculation may be used toprevent high frequencies signals from being unnecessarily attenuated.When two microphones 102 are asymmetrically located (e.g., top edge andfront face of a computing device) there may be audio content that whileperpendicular to the computing device may be perceived as off-axis. Theoff-axis perception may be due to the acoustic shadowing from the bodyof the computing device. For example, when a user is speaking straightinto a mobile phone, the front-facing microphone may capture the audiowell, but the microphone on the top edge may not capture the highfrequencies as well because they are more likely to be blocked by thebody of the mobile phone. The resulting signals captured by theasymmetrically located microphones may comprise lower frequencies thatare nearly equal and higher frequencies that may be attenuated in thetop edge microphone 102 signal relative to the front facing microphone102 signal. Other microphone 102 arrangements and angles of incidencemay further exaggerate the effect of attenuated high frequencies.

The structured noise detector 202 and channel mixer 204 described withreference to FIG. 2 may detect amplitude differences in the highfrequency components of the respective microphone signals 118 asartifacts and reduce the gain of high frequency components resulting ina slightly muffled sound. Reducing the gain, or suppressing, of the highfrequency components may result in good noise rejection at the expenseof lower fidelity. When both microphones 102 capture the voice, orsignal of interest, the CohLR measurement may indicate that themicrophone signals 118 may be correlated and that the amplitudedifferences may not be artifacts to be suppressed. In fact, thecorrelation may indicate that the high frequencies should be preserved.

The coherence calculator 302 may calculate a CohLR number, or value,that ranges from about 0 to about 1. A calculated CohLR value of one mayindicate that even if the amplitude is 20 dB higher on one microphonesignal 118 than on a second microphone signal 118, that the microphones102 have captured a common signal of interest and the amplitudedifference is not an artifact to be reduced or suppressed. When thecoherence calculator 302 calculates a CohLR value less than one, somegain reduction may occur above a threshold. Below a threshold, the CohLRmay have no effect on the calculated signal gains 208. A coherence gainadjuster 304 may adjust the signal gains 208 using the balance gains 210calculated with the balance calculator 108, the signal gains 208 and theCohLR calculated by the coherence calculator 302. The coherence gainadjuster 304 may perform similarly to the balance adjuster 112 describedabove in FIG. 1. The adjusted signal gains 208 are applied to each ofthe two or more microphone signal 118 using the gain filters 114. Thesignal gains 208 may be a combination of signal gains 208 associatedwith one or more suppression gain calculators 106. The gain filters 114may utilize the subband filter described above. Adjusting the signalgains 208 may prevent the high frequency components from beingunnecessarily reduced thereby preserving the fidelity of the outputsound field.

Further processing of the CohLR value may improve the fidelity. Forexample, the CohLR may be calculated for a given frequency bin as thecoherence between the left signal and the right signal across threefrequency bins surrounding, and including, the given frequency bin (i.e.bin+/−1). The calculated CohLR value, for example, may be almost 1 for amicrophone signal 118 that contains a harmonics. The CohLR may bevariable between about 0 and about 0.85 for noisy signals that may notbe useful to determine if two signals are correlated. The limited rangemay be resealed from 0.85 and 1 to between 0 and 1. Raising the resealedrange to the power of 4 may emphasize the desired content of highlycorrelated signals at a particular frequency. Applying additionalpsychoacoustic-based frequency and temporal smoothing may improve thefidelity further. The psychoacoustic-based smoothing may ignorefrequency and temporal components that the human ear may not perceive.

FIG. 4 is a schematic representation of yet another system formaintaining the spatial stability of a sound field when reproduced in anoutput sound field. FIG. 4 shows a system 400 that adds a signal mixer402 to the system 300. The signal mixer 402 may combine two or moreoutput signals 116 into a single mixed output signal 404. The signalmixer 402 may average the output signals 116 together or the signalmixer 402 may applied a weighted average to combine the output signals116. The system 400 may output any combination of output signals 116 andmixed output signals 404. For example, the system 400 may produce oneoutput signal 116 and one mixed output signal 404 resulting in atwo-signal output that comprises the output sound field. The system 300utilizes the coherence calculator 302 to preserve the fidelity, or highfrequency content, of the higher amplitude microphone signal 118. TheCohLR value calculated by the coherence calculator 302 may also be usedto increase the gain of the lower amplitude microphone signal 118 whenthe spectral coherence is relatively high. Normalizing the amplitude ofthe two or more microphone signals 118 may allow beam forming of two ormore microphone signals 118 to be based on time differences and notamplitude differences. Any signal content that is highly correlatedacross the two microphones signals 118 may be enhanced, and any signalcontent that is not well correlated is either not enhanced or may besignificantly reduced. The signal mixer 402 may perform beam forming inaddition to combining two or more output signals 116 together.

FIG. 5 is a schematic representation of a still further system formaintaining the spatial stability of a sound field when reproduced inthe output sound field. The system of FIG. 5 may provide the same orsimilar functionality as the systems described with reference to FIG. 1,FIG. 2 and FIG. 3. FIG. 5 does not show the background noise estimator104, the structured noise detector 202, the channel mixer 204, the gainadjuster 206 and the coherence gain adjuster 304 but they may beincluded in the system 500. The systems 100, 200 and 300 described abovemay enhance a sound field captured by two or more microphones 102. Thesystem 500 includes a receiver 502 that may receive an audio signalrepresenting, for example, a far side conversation. The received audiosignal content, for example the far side conversation, may be reproducedusing an audio transducer 504 that may be within range to be captured bytwo or more microphones 102. A system such as, for example, system 300may enhance the captured far side conversation instead of suppressingthe recaptured audio, or echo. The correlated recaptured audio, or echo,using two or more microphones 102 may not be suppressed because thecoherence calculator 302 may indicate that the recaptured audio may be asignal of interest resulting in enhancement of the undesirable echo.

The receiver 502 may receive a far side audio signal from anothercomputing device or other similar audio source. The receiver 502 may beconnected to a wireless or wired network. The far side audio signal maybe reproduced using the audio transducer 504. The microphones 102 mayrecapture the far side audio signal reproduced using the audiotransducer 504. The recaptured far side audio signal may be perceived asan echo. When the echo is correlated on any two or more of themicrophones the coherence calculator 302 may indicate that the echo is asignal of interest that may result in the echo being enhanced. The echomay be considered an undesirable signal component to be removed. An echofilter 506 may calculate a predicted echo (D) 508 that when applied tothe microphone signals 118 may reduce the echo. In one implementation,echo noise may be reduced using the system for fast echo cancellationdisclosed in U.S. Pat. No. 8,036,879, which is incorporated herein byreference, except that in the event of any inconsistent disclosure ordefinition from the present specification, the disclosure or definitionherein shall be deemed to prevail. The echo filter 506 and the coherencecalculator 302 may indicate opposite gain values to be applied to themicrophone signal 118 (Y) where the echo filter 506 may indicate thatthe gain should be reduced and the coherence calculator 302 may indicatethat the gain should be increased. In some cases, the echo may beenhanced. A coherence echo calculator 510 may calculate a pair-wisespectral coherence, or a pair-wise echo spectral coherence, CohDY thatmay be used as an indicator of a correlation between the predicted echo(D) and the observed microphone signal (Y). The coherence echocalculator 510 may receive both the predicted echo (D) 508 and themicrophone signal 118. A strong correlation between the predicted echo(D) 506 and the microphone signal 118 (Y) may indicate that the higheramplitude microphone signal 118 should not be preserved and the loweramplitude microphone signal 118 should not be increased.

A coherence echo gain adjuster 512 may adjust the signal gains 208 usingthe balance gains 210, the signal gains 208, the CohLR and the CohDYcalculated by the coherence echo calculator 510. The coherence echo gainadjuster 512 may perform similarly to the balance adjuster 112 describedabove with reference to FIG. 1. The CohLR value may be multiplied by1−CohDY and the product applied to the signal gains 208 in a similarfashion described above in reference to the coherence gain adjuster 304.Using both of the CohLR and the CohDY values in order to adjust thesignal gains 208 may reduce the noise artifacts, preserve and enhancethe signal of interest, and reduce the echo. The adjusted signal gains208 are applied to each of the two or more microphone signal 118 usingthe gain filters 114. The signal gains 208 may be a combination ofsignal gains 208 associated with one or more suppression gaincalculators 106. The gain filters 114 may utilize the subband filterdescribed above.

FIG. 6 is a representation of a method for maintaining the spatialstability of the sound field. The method 600 may be, for example,implemented using the systems 200 described herein with reference toFIG. 2. The method 600 includes the act of calculating balance gains foreach of two or more microphone signals 602. The balance gain may beassociated with a spatial image in the sound field. One or more signalvalues may be calculated for each of the two or more microphone signals604. The signal values may be the background noise estimate or signalgains associated with echo cancellation and noise reduction processes.Structured noise content may be detected for each of the two or moremicrophone signals 606. The structured noise content may be for example,impulse noise or tonal noise. A first microphone signal of the two ormore microphone signals may be mixed with a second microphone signal ofthe two or more microphone signals responsive to the detected structurednoise 608. Increasing amounts of detected structured noise may increasethe amount of mixing, or blending, of the first microphone signal withthe second microphone signal. The gain may be adjusted for the two ormore microphone signals, including the mixed first microphone signal andsecond microphone signal, responsive to the calculated balance gains andthe one or more signal values for each of the two or more microphonesignals 610.

FIG. 7 is a schematic representation of a system for maintaining thespatial stability of the sound field. The system 700 comprises aprocessor 702, memory 704 (the contents of which are accessible by theprocessor 702) and an I/O interface 706. The memory 704 may storeinstructions which when executed using the process 702 may cause thesystem 700 to render the functionality associated with maintaining thespatial stability of the sound field as described herein. For example,the memory 704 may store instructions which when executed using theprocessor 702 may cause the system 700 to render the functionalityassociated with the background noise estimator 104, the suppression gaincalculator 106, the balance calculator 108, the difference limiter 110,the gain filter 114, the structured noise detector 202, the channelmixer 204 and the gain adjuster 206 as described herein. In addition,data structures, temporary variables and other information may storedata in data storage 708.

The processor 702 may comprise a single processor or multiple processorsthat may be disposed on a single chip, on multiple devices ordistributed over more that one system. The processor 702 may be hardwarethat executes computer executable instructions or computer code embodiedin the memory 704 or in other memory to perform one or more features ofthe system. The processor 702 may include a general purpose processor, acentral processing unit (CPU), a graphics processing unit (GPU), anapplication specific integrated circuit (ASIC), a digital signalprocessor (DSP), a field programmable gate array (FPGA), a digitalcircuit, an analog circuit, a microcontroller, any other type ofprocessor, or any combination thereof.

The memory 704 may comprise a device for storing and retrieving data,processor executable instructions, or any combination thereof. Thememory 704 may include non-volatile and/or volatile memory, such as arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM), or a flash memory. The memory 704may comprise a single device or multiple devices that may be disposed onone or more dedicated memory devices or on a processor or other similardevice. Alternatively or in addition, the memory 704 may include anoptical, magnetic (hard-drive) or any other form of data storage device.

The memory 704 may store computer code, such as the background noiseestimator 104, the suppression gain calculator 106, the balancecalculator 108, the difference limiter 110, the gain filter 114, thestructured noise detector 202, the channel mixer 204 and the gainadjuster 206 as described herein. The computer code may includeinstructions executable with the processor 702. The computer code may bewritten in any computer language, such as C, C++, assembly language,channel program code, and/or any combination of computer languages. Thememory 704 may store information in data structures including, forexample, suppression gains.

The I/O interface 706 may be used to connect devices such as, forexample, the microphones 102, to other components of the system 700.

All of the disclosure, regardless of the particular implementationdescribed, is exemplary in nature, rather than limiting. The system 700may include more, fewer, or different components than illustrated inFIG. 7. Furthermore, each one of the components of system 700 mayinclude more, fewer, or different elements than is illustrated in FIG.7. Flags, data, databases, tables, entities, and other data structuresmay be separately stored and managed, may be incorporated into a singlememory or database, may be distributed, or may be logically andphysically organized in many different ways. The components may operateindependently or be part of a same program or hardware. The componentsmay be resident on separate hardware, such as separate removable circuitboards, or share common hardware, such as a same memory and processorfor implementing instructions from the memory. Programs may be parts ofa single program, separate programs, or distributed across severalmemories and processors.

FIG. 8 is a representation of a method for maintaining the spatialstability of the sound field. The method 800 may be, for example,implemented using the systems 300 described herein with reference toFIG. 3. The method 800 includes the act of calculating balance gains foreach of two or more microphone signals 802. The balance gain may beassociated with a spatial image in the sound field. One or more signalvalues may be calculated for each of the two or more microphone signals804. The signal values may be the background noise estimate or signalgains associated with echo cancellation and noise reduction processes. Apair-wise spectral coherence may be calculated between each of the twoor more microphone signals 806. The pair-wise spectral coherence mayindicate that two or more microphone signals are correlated and may havecaptured a signal of interest. The two or more microphone signals may begain adjusted responsive to the calculated balance gains, the one ormore signal values, and the pair-wise spectral coherence for each of thetwo or more microphone signals 808. The spectral coherence value may beused to prevent high amplitude high frequencies signals from beingunnecessarily attenuated and may also be used to increase the gain oflow amplitude high frequency signals.

FIG. 9 is a schematic representation of a system for maintaining thespatial stability of the sound field. The system 900 comprises aprocessor 902, memory 904 (the contents of which are accessible by theprocessor 902) and an I/O interface 906. The memory 904 may storeinstructions which when executed using the process 902 may cause thesystem 900 to render the functionality associated with maintaining thespatial stability of the sound field as described herein. For example,the memory 904 may store instructions which when executed using theprocessor 902 may cause the system 900 to render the functionalityassociated with the background noise estimator 104, the suppression gaincalculator 106, the balance calculator 108, the difference limiter 110,the gain filter 114, the coherence calculator 302, the coherence gainadjuster 304 and the signal mixer 402 as described herein. In addition,data structures, temporary variables and other information may storedata in data storage 908.

The processor 902 may comprise a single processor or multiple processorsthat may be disposed on a single chip, on multiple devices ordistributed over more than one system. The processor 902 may be hardwarethat executes computer executable instructions or computer code embodiedin the memory 904 or in other memory to perform one or more features ofthe system. The processor 902 may include a general purpose processor, acentral processing unit (CPU), a graphics processing unit (GPU), anapplication specific integrated circuit (ASIC), a digital signalprocessor (DSP), a field programmable gate array (FPGA), a digitalcircuit, an analog circuit, a microcontroller, any other type ofprocessor, or any combination thereof.

The memory 904 may comprise a device for storing and retrieving data,processor executable instructions, or any combination thereof. Thememory 904 may include non-volatile and/or volatile memory, such as arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM), or a flash memory. The memory 904may comprise a single device or multiple devices that may be disposed onone or more dedicated memory devices or on a processor or other similardevice. Alternatively or in addition, the memory 904 may include anoptical, magnetic (hard-drive) or any other form of data storage device.

The memory 904 may store computer code, such as the background noiseestimator 104, the suppression gain calculator 106, the balancecalculator 108, the difference limiter 110, the gain filter 114, thecoherence calculator 302, the coherence gain adjuster 304 and the signalmixer 402 as described herein. The computer code may includeinstructions executable with the processor 902. The computer code may bewritten in any computer language, such as C, C++, assembly language,channel program code, and/or any combination of computer languages. Thememory 904 may store information in data structures including, forexample, suppression gains.

The I/O interface 906 may be used to connect devices such as, forexample, the microphones 902, to other components of the system 900. Thesystem 900 may include more, fewer, or different components thanillustrated in FIG. 9. Furthermore, each one of the components of system900 may include more, fewer, or different elements than is illustratedin FIG. 9. Flags, data, databases, tables, entities, and other datastructures may be separately stored and managed, may be incorporatedinto a single memory or database, may be distributed, or may belogically and physically organized in many different ways. Thecomponents may operate independently or be part of a same program orhardware. The components may be resident on separate hardware, such asseparate removable circuit boards, or share common hardware, such as asame memory and processor for implementing instructions from the memory.Programs may be parts of a single program, separate programs, ordistributed across several memories and processors.

FIG. 10 is a representation of a method for maintaining the spatialstability of the sound field. The method 1000 may be, for example,implemented using the systems 500 described herein with reference toFIG. 5. The method 1000 includes the act of calculating balance gainsfor each of two or more microphone signals 1002. The balance gain may beassociated with a spatial image in the sound field. One or more signalvalues may be calculated for each of the two or more microphone signals1004. The signal values may be the background noise estimate or signalgains associated with echo cancellation and noise reduction processes. Apredicted echo may be calculated for a received audio signal 1006. Thepredicted echo may be used to reduce an echo signal. A pair-wise echospectral coherence may be calculated between the predicted echo and thetwo or more microphone signals 1008. The pair-wise echo spectralcoherence may indicate that the predicted echo is correlated to one ormore of the captured two or more microphone signals. A pair-wisespectral coherence between each of the two or more microphone signals1010. The pair-wise spectral coherence may indicate that two or moremicrophone signals are correlated and may have captured a signal ofinterest. The two or more microphone signals may be gain adjustedresponsive to the calculated balance gains, the one or more signalvalues, the echo spectral coherence and the pair-wise spectral coherencefor each of the two or more microphone signals 1012. Using both of theecho spectral coherence and the spectral coherence values in order toadjust the signal gains may reduce the noise artifacts, preserve andenhance the signal of interest, and reduce the echo.

FIG. 11 is a schematic representation of a system for maintaining thespatial stability of the sound field. The system 1100 comprises aprocessor 1102, memory 1104 (the contents of which are accessible by theprocessor 1102) and an I/O interface 1106. The memory 1104 may storeinstructions which when executed using the process 1102 may cause thesystem 1100 to render the functionality associated with maintaining thespatial stability of the sound field as described herein. For example,the memory 1104 may store instructions which when executed using theprocessor 1102 may cause the system 1100 to render the functionalityassociated with the background noise estimator 104, the suppression gaincalculator 106, the balance calculator 108, the difference limiter 110,the gain filter 114, the coherence calculator 302, the echo filter 506,the coherence echo calculator 510 and the coherence echo gain adjuster512 as described herein. In addition, data structures, temporaryvariables and other information may store data in data storage 1108.

The processor 1102 may comprise a single processor or multipleprocessors that may be disposed on a single chip, on multiple devices ordistributed over more that one system. The processor 1102 may behardware that executes computer executable instructions or computer codeembodied in the memory 1104 or in other memory to perform one or morefeatures of the system. The processor 1102 may include a general purposeprocessor, a central processing unit (CPU), a graphics processing unit(GPU), an application specific integrated circuit (ASIC), a digitalsignal processor (DSP), a field programmable gate array (FPGA), adigital circuit, an analog circuit, a microcontroller, any other type ofprocessor, or any combination thereof.

The memory 1104 may comprise a device for storing and retrieving data,processor executable instructions, or any combination thereof. Thememory 1104 may include non-volatile and/or volatile memory, such as arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM), or a flash memory. The memory1104 may comprise a single device or multiple devices that may bedisposed on one or more dedicated memory devices or on a processor orother similar device. Alternatively or in addition, the memory 1104 mayinclude an optical, magnetic (hard-drive) or any other form of datastorage device.

The memory 1104 may store computer code, such as the background noiseestimator 104, the suppression gain calculator 106, the balancecalculator 108, the difference limiter 110, the gain filter 114, thecoherence calculator 302, the echo filter 506, the coherence echocalculator 510 and the coherence echo gain adjuster 512 as describedherein. The computer code may include instructions executable with theprocessor 1102. The computer code may be written in any computerlanguage, such as C, C++, assembly language, channel program code,and/or any combination of computer languages. The memory 1104 may storeinformation in data structures including, for example, suppressiongains.

The I/O interface 1106 may be used to connect devices such as, forexample, the microphones 102, the receiver 502 and the audio transducer504 to other components of the system 900. The system 1100 may includemore, fewer, or different components than illustrated in FIG. 11.Furthermore, each one of the components of system 1100 may include more,fewer, or different elements than is illustrated in FIG. 11. Flags,data, databases, tables, entities, and other data structures may beseparately stored and managed, may be incorporated into a single memoryor database, may be distributed, or may be logically and physicallyorganized in many different ways. The components may operateindependently or be part of a same program or hardware. The componentsmay be resident on separate hardware, such as separate removable circuitboards, or share common hardware, such as a same memory and processorfor implementing instructions from the memory. Programs may be parts ofa single program, separate programs, or distributed across severalmemories and processors.

The functions, acts or tasks illustrated in the figures or described maybe executed in response to one or more sets of logic or instructionsstored in or on computer readable media. The functions, acts or tasksare independent of the particular type of instructions set, storagemedia, processor or processing strategy and may be performed bysoftware, hardware, integrated circuits, firmware, micro code and thelike, operating alone or in combination. Similarly, the microphones maycomprise devices that convert sound into signals (e.g., electricalsignals) and may include hardware that converts the signal output intodigital data. Likewise, processing strategies may includemultiprocessing, multitasking, parallel processing, distributedprocessing, and/or any other type of processing. In one embodiment, theinstructions are stored on a removable media device for reading by localor remote systems. In other embodiments, the logic or instructions arestored in a remote location for transfer through a computer network orover telephone lines. In yet other embodiments, the logic orinstructions may be stored within a given computer such as, for example,a CPU.

While various embodiments of the system and method for on-demand usercontrol have been described, it will be apparent to those of ordinaryskill in the art that many more embodiments and implementations arepossible within the scope of the present invention. Accordingly, theinvention is not to be restricted except in light of the attached claimsand their equivalents.

1. A computer implemented method for maintaining spatial stability of asound field comprising: calculating balance gains for each of two ormore microphone signals; calculating one or more signal values for eachof the two or more microphone signals; detecting structured noisecontent for each of the two or more microphone signals; mixing a firstmicrophone signal of the two or more microphone signals with a secondmicrophone signal of the two or more microphone signals responsive tothe detected structured noise; and gain adjusting the two or moremicrophone signals, including the mixed first microphone signal andsecond microphone signal, responsive to the calculated balance gains andthe one or more signal values for each of the two or more microphonesignals.
 2. The computer implemented method of claim 1, where the one ormore signal values comprises one or more of an estimated backgroundnoise and a calculated suppression gain.
 3. The computer implementedmethod of claim 2 where the suppression gain comprises one or more of anoise reduction calculation and an echo cancellation calculation.
 4. Thecomputer implemented method of claim 3, where the noise reductioncalculation comprises any one or more of a wind noise reductioncalculation, transients noise reduction calculation, a road noisereduction calculation, repetitive noise reduction calculation and anengine noise reduction calculation.
 5. The computer implemented methodof claim 1, where the structured noise content includes any one or moreof tonal noise and impulse noise.
 6. The computer implemented method ofclaim 1, where increasing amounts of detected structured noise contentincreases the amount of mixing of the first microphone signal with thesecond microphone signal.
 7. The computer implemented method of claim 6,where an amount of detected structured noise exceeding a threshold mixesthe entire first microphone signal with the second microphone signal. 8.The computer implemented method of claim 7, where the second microphonesignal has higher amplitude than the first microphone signal.
 9. Thecomputer implemented method of claim 1, where mixing a first microphonesignal of the two or more microphone signals with a second microphonesignal of the two or more microphone signals comprises mixing on afrequency bin-by-frequency bin basis.
 10. The computer implementedmethod of claim 1, further comprising generating a set of sub-bands foreach of the two or more microphone signals using a subband filter or aFast Fourier Transform.
 11. The computer implemented method of claim 1,further comprising generating a set of sub-bands for each of the two ormore microphone signals according to a critical, octave, mel, or barkband spacing technique.
 12. A system for maintaining spatial stabilityof a sound field comprising: a balance calculator to calculate balancegains for each of two or more microphone signals; two or more signalvalue generators, each one associated with one of the two or moremicrophone signals, to calculate one or more signal values; a structurednoise detector to detect structured noise content for each of the two ormore microphone signals; a channel mixer to mix a first microphonesignal of the two or more microphone signals with a second microphonesignal of the two or more microphone signals responsive to the detectedstructured noise; and a gain adjuster to adjust the two or moremicrophone signals, including the mixed first microphone signal andsecond microphone signal, responsive to the calculated balance gains andthe one or more signal values for each of the two or more microphonesignals.
 13. The system for maintaining spatial stability of a soundfield of claim 12, where the one or more signal values comprises one ormore of an estimated background noise and a calculated suppression gain.14. The system for maintaining spatial stability of a sound field ofclaim 13 where the suppression gain comprises one or more of a noisereduction calculation and an echo cancellation calculation.
 15. Thesystem for maintaining spatial stability of a sound field of claim 14,where the noise reduction calculation comprises any one or more of awind noise reduction calculation, transients noise reductioncalculation, a road noise reduction calculation, a repetitive noisereduction calculation and an engine noise reduction calculation.
 16. Thesystem for maintaining spatial stability of a sound field of claim 12,where the structured noise content includes any one or more of tonalnoise and impulse noise.
 17. The system for maintaining spatialstability of a sound field of claim 12, where increasing amounts ofdetected structured noise content increases the amount of mixing of thefirst microphone signal with the second microphone signal by the channelmixer.
 18. The system for maintaining spatial stability of a sound fieldof claim 17, where when an amount of detected structured noise exceeds athreshold, the channel mixer mixes the entire first microphone signalwith the second microphone signal.
 19. The system for maintainingspatial stability of a sound field of claim 18, where the secondmicrophone signal has higher amplitude than the first microphone signal.20. The system for maintaining spatial stability of a sound field ofclaim 12, where mixing a first microphone signal of the two or moremicrophone signals with a second microphone signal of the two or moremicrophone signals comprises mixing on a frequency bin-by-frequency binbasis.
 21. The system for maintaining spatial stability of a sound fieldof claim 12, further comprising generating a set of sub-bands for eachof the two or more microphone signals using a subband filter or a FastFourier Transform.
 22. The system for maintaining spatial stability of asound field of claim 12, further comprising generating a set ofsub-bands for each of the two or more microphone signals according to acritical, octave, mel, or bark band spacing technique.